Fluoropolymers are applied to a wide number of substrates in order to confer release, chemical and heat resistance, corrosion protection, cleanability, low flammability, and weatherability. Coatings of polytetrafluoroethylene (PTFE) homopolymers and modified PTFE provide the highest heat stability among the fluoropolymers, but unlike tetrafluoroethylene (TFE) copolymers, cannot be melt processed to form films and coatings. Therefore, other processes have been developed for applying coatings of PTFE homopolymers and modified PTFE. One such process is dispersion coating which applies the fluoropolymer in dispersion form. Dispersions used in coating processes are usually in a concentrated form and contain a significant quantity of nonionic surfactant, e.g. 6-8 weight percent, as taught in Marks et al., U.S. Pat. No. 3,037,953, and in Holmes, U.S. Pat. No. 3,704,272 to raise the solids from nominally 35 wt % in the raw dispersion to about 60 wt % in the concentrated dispersion. Miura et al., U.S. Pat. No. 6,153,688 discloses a similar process. Similar dispersions and coating processes are also used for making coatings of melt-processible fluoropolymers.
Fluorosurfactants are typically used as an ingredient in the dispersion polymerization of fluoropolymers since a non-telogenic dispersing agent is generally required in commercial fluoropolymer dispersion polymerization processes. For example, an early description of fluorosurfactants used commercially is found in U.S. Pat. No. 2,559,752 to Berry. These fluorosurfactants are anionic surfactants, usually perfluorinated carboxylic acids, e.g., ammonium perfluorocaprylate or ammonium perfluorooctanoate.
Because of environmental concerns and because anionic fluorosurfactants are expensive, processes have been developed for the removal of anionic fluorosurfactants from aqueous fluoropolymer dispersions. One common method is to remove fluorosurfactant by adsorption onto an anion exchange resin as taught in U.S. Pat. No. 3,882,153 (Seki et al) and U.S. Pat. No. 4,282,162 (Kuhls) and U.S. Pat. No. 6,833,403 (Bladel et al.). For effective removal, such dispersions are stabilized with a non-ionic surfactant, such as alkyl phenol ethoxylates or aliphatic alcohol ethoxylates as disclosed in U.S. Pat. No. 3,037,953 to Marks et al., U.S. Pat. No. 6,153,688 to Miura et al., and U.S. 2003/0130393 to Cavanaugh et al. Dispersions stabilized with nonionic surfactant are used since removal of the fluorosurfactant without nonionic surfactant being present generally results in coagulation of the dispersion.
Although anion exchange processes for fluorosurfactant reduction can be carried out using a slurry method where the anion exchange resin is contacted with the dispersion in a stirred vessel followed by filtering to remove the anion exchange resin, fixed bed processes are often desirable for commercial operation. In a fixed bed process, the anion exchange resin is held in a container, typically cylindrical in shape and referred to as a column, and the dispersion is passed though the column either by gravity feed or by pumping. When pumped, flow can be upward or downward through the bed.
However, in fixed bed anion exchange processes with some dispersions, especially those with high solids levels, e.g., over 45 wt %, and/or dispersions with high molecular weight fluoropolymer, a significant dispersion viscosity increase can occur in the column as the fluorosurfactant content is reduced. A dispersion viscosity increase will correspondingly increase the pressure drop across the column. In gravity feed columns, if the pressure drop exceeds head pressure, dispersion flow will stop. In columns with a pumped dispersion feed, flow can be forced through the column but another type of failure often referred to as “fingering” may result. With fingering, a high viscosity dispersion matrix forms blocked regions and flow occurs only in fingers between these regions. With only a portion of the bed being utilized, premature PFOA breakthrough (i.e. dispersion with high fluorosurfactant levels) can occur. When fingering occurs, the ion exchange bed typically must be replaced.